[Technical Field]
[0001] The present invention relates to a superabsorbent polymer exhibiting more improved
absorption rate and liquid permeability as well as excellent basic absorption performances,
and a preparation method thereof.
[Background]
[0002] A superabsorbent polymer (SAP) is a synthetic polymeric material capable of absorbing
moisture from about 500 to 1000 times its own weight. Various manufacturers have given
it different names, such as SAM (Super Absorbency Material), AGM (Absorbent Gel Material),
etc. Such superabsorbent polymers started to be practically applied in sanitary products,
and now they are widely used not only for hygiene products such as disposable diapers
for children, etc., but also for water retaining soil products for gardening, water
stop materials for civil engineering and construction, sheets for raising seedling,
fresh-keeping agents for food distribution fields, materials for poultices, etc.
[0003] In most cases, these superabsorbent polymers have been widely used in the field of
hygienic materials such as diapers, sanitary napkins, etc. For these applications,
superabsorbent polymers are required to exhibit high absorbency with respect to water,
etc., must not release absorbed water even under an external pressure, and also must
maintain their shape under volume expansion (swelling) after water absorption to show
excellent permeability.
[0004] Recently, with increasing demand for thin diapers, the content of fibrous materials
such as pulp, etc. tends to decrease and the proportion of the superabsorbent polymer
tends to relatively increase in the diapers. Therefore, the superabsorbent polymer
needs to have the performance of the fibrous materials in the diapers. To achieve
this, the superabsorbent polymer should have high absorption rate and liquid permeability
as well as high absorbency. Particularly, as diapers become thinner, there is an increasing
concern about leakage of the urine from the diaper according to the movement of a
baby who is the user of the diaper, and accordingly, the superabsorbent polymer is
increasingly required to have the high absorption rate.
[0005] Meanwhile, in order for the superabsorbent polymer to exhibit the above-mentioned
high liquid permeability, even after the superabsorbent polymer particles are swollen
by absorbing moisture, the superabsorbent polymer needs to retain in its shape, thereby
maintaining voids between particles. This is because voids between the particles act
as a flow path to ensure excellent liquid permeability of the superabsorbent polymer.
For this reason, in order to provide a superabsorbent polymer exhibiting more improved
liquid permeability and other excellent physical properties, such a superabsorbent
polymer needs to exhibit a higher gel strength through surface crosslinking, etc.
[0006] Further, to exhibit the higher absorption rate, the superabsorbent polymer needs
to have a porous structure having a large surface area and a large number of pores
formed inside thereof. Previously, superabsorbent polymers having such a porous structure
have been prepared by applying a foaming agent, etc. However, this superabsorbent
polymer is likely to be uneven in the particle shape after pulverization. Therefore,
when surface crosslinking is performed after pulverization or when an additive, etc.
is mixed to improve physical properties, uneven surface crosslinking may occur or
uneven coating of the additive may occur in many cases. As a result, in the previous
technology of realizing the high absorption rate of the superabsorbent polymer by
forming the porous structure, etc., other physical properties such as liquid permeability,
absorption performances, etc. have been frequently deteriorated.
[0007] Accordingly, there is a continuous demand for the development of a technology capable
of providing a superabsorbent polymer exhibiting more improved liquid permeability
and absorption rate while maintaining excellent absorption performances.
[0008] EP 3 249 001 A1 relates to a super absorbent resin, and the super absorbent resin can exhibit a fast
absorption rate and high gel strength even in a partially swollen state through the
size optimization of partially swollen gel particles.
[0009] EP3 192 826 A1 discloses the preparation method of superabsorbent polymer which can increase suction
power without degradation of other properties of superabsorbent polymer, and thus,
the prepared superabsorbent polymer may be usefully used as material of hygienic goods
such as a diaper.
[0010] US 2014/312 273 A1 discloses a method of preparing superabsorbing polymer composition comprising steps
of (i-ii) obtaining water-insoluble, aqueous untreated hydrogel polymer by free-radical
polymerization of ethylenically unsaturated monomers containing acid groups or salts
thereof and cross-linkers as well as blowing agents, (iii) drying, grinding and sieving
the hydrogel polymer to defined size and (v-vi) surface crosslinking the ground and
sieved hydrogel polymer followed by drying and finishing steps to achieve the water
absorbing composition.
[0011] EP 2 518 092 A1 discloses a water-absorbable polyacrylic acid resin powder and a process for production
thereof, wherein the dried and grinded acrylic acid-based crosslinked hydrogel fine
particles are further surface crosslinked.
[Technical Problem]
[0012] The present invention provides a superabsorbent polymer exhibiting more improved
absorption rate and liquid permeability as well as excellent basic absorption performances,
and a preparation method thereof.
[Technical Solution]
[0013] The present invention provides a superabsorbent polymer including:
a base polymer powder including a first crosslinked polymer of water-soluble ethylene-based
unsaturated monomers having acidic groups which are at least partially neutralized;
and
a surface crosslinked layer which is formed on the base polymer powder and includes
a second cross-linked polymer in which the first crosslinked polymer is additionally
crosslinked via a surface crosslinking agent, wherein the superabsorbent polymer includes
10% by number or more of superabsorbent polymer particles each particle having an
aspect ratio of less than 0.5, the aspect ratio defined as the shortest diameter/the
longest diameter of the superabsorbent polymer particle, and the superabsorbent polymer
has a saline (0.685% by weight of an aqueous solution of sodium chloride) flow conductivity
(SFC; ·10-7cm3·s/g) of 30(·10-7cm3·s/g) or more, and
wherein the surface crosslinking agent includes two or more kinds of alkylene carbonates
having 2 to 5 carbon atoms, in which the two or more kinds of the alkylene carbonates
have different carbon numbers.
[0014] Further, the present invention provides a method of preparing the superabsorbent
polymer, the method including the steps of:
carrying out a crosslinking polymerization of water-soluble ethylene-based unsaturated
monomers having acidic groups which are at least partially neutralized, in the presence
of a foaming agent and an internal crosslinking agent to form a water-containing gel
polymer including a first cross-linked polymer;
gel-pulverizing, drying, pulverizing, and size-sorting the water-containing gel polymer
to form a base polymer including 10% by number or more of base polymer powder having
an aspect ratio of less than 0.5, the aspect ratio defined as the shortest diameter/the
longest diameter; measured by electron microscopy and
carrying out a surface crosslinking of the base polymer by heat treatment in the presence
of a surface crosslinking liquid containing a surface crosslinking agent and a liquid
medium and having a surface tension of 30 mN/m to 50 mN/m at a temperature of 23±0.5
°C and a humidity of 45±0.5%, as measured by a tensiometer,
wherein the surface crosslinking agent includes two or more kinds of alkylene carbonates
having 2 to 5 carbon atoms, in which the two or more kinds of the alkylene carbonates
have different carbon numbers, and
wherein the foaming agent is carbonate and added in a concentration of 0.01 parts
by weight to 1.0 parts by weight with respect to 100 parts by weight of the acrylic
acid-based monomer.
[0015] Hereinafter, a superabsorbent polymer according to specific embodiments of the present
invention and a preparation method thereof will be described in more detail.
[0016] Additionally, unless stated otherwise throughout this specification, the term "comprising"
or "including" means to include any element (or component) without particular limitation,
and it may not be interpreted as a meaning of excluding addition of another element
(or component).
[0017] According to one embodiment of the present invention, the present invention provides
a superabsorbent polymer including a base polymer powder including a first crosslinked
polymer of water-soluble ethylene-based unsaturated monomers having acidic groups
which are at least partially neutralized; and a surface crosslinked layer which is
formed on the base polymer powder and includes a second cross-linked polymer in which
the first crosslinked polymer is additionally crosslinked via a surface crosslinking
agent, wherein the superabsorbent polymer includes 10% by number or more of superabsorbent
polymer particles each particle having an aspect ratio of less than 0.5, the aspect
ratio defined as the shortest diameter/the longest diameter of the superabsorbent
polymer particle, measured by electron microscopy and the superabsorbent polymer has
a saline (0.685% by weight of an aqueous solution of sodium chloride) flow conductivity
(SFC; ·10
-7cm
3·s/g) of 30 (·10
-7cm
3·s/g) or more, and wherein the surface crosslinking agent includes two or more kinds
of alkylene carbonates having 2 to 5 carbon atoms, in which the two or more kinds
of the alkylene carbonates have different carbon numbers.
[0018] As a result of continuous studies, the present inventors found that when particles
having a high aspect ratio are obtained in a predetermined level or more in the presence
of a foaming agent, etc. during a crosslinking polymerization according to the preparation
method described below, and then a surface crosslinking process is performed using
a surface crosslinking liquid having a reduced surface tension, it is possible to
prepare and provide a superabsorbent polymer having improved liquid permeability and
absorption rate as well as excellent basic absorbency, thereby completing the present
invention.
[0019] Basically, since the superabsorbent polymer of one embodiment may be obtained by
foaming polymerization using a foaming agent, etc. during the polymerization process,
the base polymer powder and superabsorbent polymer particles after pulverization may
be allowed to have a low aspect ratio and a large surface area. For example, the superabsorbent
polymer may be prepared to include 10% by number or more, 10% by number to 60% by
number, or 10% by number to 50% by number of the superabsorbent polymer particles
each particle having an aspect ratio of less than 0.5, the aspect ratio defined as
the shortest diameter/the longest diameter of the superabsorbent polymer particle.
[0020] As such, during the preparation process of the superabsorbent polymer, the base polymer
powder and superabsorbent polymer particles are obtained such that they include particles
having a low aspect ratio at a predetermined level or more and their surface area
is increased, and as a result, the superabsorbent polymer of one embodiment may exhibit
a higher absorption rate, etc.
[0021] However, when particles having a low aspect ratio are formed at a predetermined level
or more, the shape of the particles is uneven, and thus it is difficult to evenly
perform the subsequent surface crosslinking. As a result, it is difficult to improve
absorbency under pressure and liquid permeability of the superabsorbent polymer at
the same time. This is because surface crosslinking of particles having a low aspect
ratio unevenly occurs, as compared with that of particles having an aspect ratio close
to 1.
[0022] However, according to continuous experimental results of the present inventors, it
was found that when a surface crosslinking liquid having a relatively low surface
tension is obtained by a method described below, and then surface crosslinking is
performed using the same, a surface crosslinked layer having excellent surface crosslinking
degree and strength may be evenly formed on the base polymer powder including the
particles having a low aspect ratio at a predetermined level or more. This is presumably
because the penetration of the surface crosslinking liquid may be relatively shallow
and uniformly controlled.
[0023] Accordingly, the superabsorbent polymer of one embodiment may exhibit the excellent
absorption rate and more improved liquid permeability and absorbency under pressure.
The improved liquid permeability of the superabsorbent polymer of one embodiment may
be defined by the above range of SFC.
[0024] Therefore, unlike conventional common sense that it is difficult to improve the absorption
rate and liquid permeability at the same time, the superabsorbent polymer of one embodiment
may exhibit both more improved absorption rate and liquid permeability while maintaining
excellent basic absorption performances, thereby being suitably applied to sanitary
materials such as thinner diapers, etc.
[0025] Hereinafter, the superabsorbent polymer of one embodiment will be described in more
detail.
[0026] The 'superabsorbent polymer', as used herein, refers to a superabsorbent polymer
including a base polymer powder including a first crosslinked polymer of water-soluble
ethylene-based unsaturated monomers having acidic groups which are at least partially
neutralized; and a surface crosslinked layer which is formed on the base polymer powder
and includes a second cross-linked polymer in which the first crosslinked polymer
is additionally crosslinked via a surface crosslinking agent.
[0027] The water-soluble ethylene-based unsaturated monomer may be any monomer commonly
used in the preparation of superabsorbent polymers. For non-limiting example, the
water-soluble ethylene-based unsaturated monomer may be a compound represented by
the following Chemical Formula 1:
[Chemical Formula 1] R
1-COOM
1
in Chemical Formula 1, R
1 is an alkyl group having 2 to 5 carbon atoms and containing an unsaturated bond,
and
M
1 is a hydrogen atom, a monovalent or divalent metal, an ammonium group, or an organic
amine salt.
[0028] Appropriately, the monomer may be one or more selected from the group consisting
of acrylic acid, methacrylic acid, and a monovalent metal salt, a divalent metal salt,
an ammonium salt, and an organic amine salt thereof. When acrylic acid or a salt thereof
is used as the water-soluble ethylene-based unsaturated monomer, it is advantageous
in terms of obtaining the superabsorbent polymer having improved absorbency. In addition,
the monomer may include one or more selected from the group consisting of an anionic
monomer such as maleic anhydride, fumaric acid, crotonic acid, itaconic acid, 2-acryloylethane
sulfonic acid, 2-methacryloylethane sulfonic acid, 2-(meth)acryloylpropane sulfonic
acid, or 2-(meth)acrylamide-2-methyl propane sulfonic acid, and salts thereof; a nonionic
hydrophilic monomer such as (meth)acrylamide, N-substituted (meth)acrylate, 2-hydroxyethyl(meth)acrylate,
2-hydroxypropyl(meth)acrylate, methoxy polyethylene glycol (meth)acrylate, or polyethylene
glycol (meth)acrylate; and an amino group-containing unsaturated monomer such as (N,N)-dimethylaminoethyl(meth)acrylate
or (N,N)-dimethylaminopropyl(meth)acrylate, and a quaternary compound thereof.
[0029] Here, the water-soluble ethylene-based unsaturated monomer may have acidic groups
which are at least partially neutralized. Preferably, those partially neutralized
with an alkali substance such as sodium hydroxide, potassium hydroxide, ammonium hydroxide
or the like may be used.
[0030] In this regard, a neutralization degree of the monomer may be about 40 mol% to 95
mol%, or 40 mol% to 80 mol%, or 45 mol% to 75 mol%. The range of the neutralization
degree may vary depending on the final physical properties. An excessively high degree
of neutralization renders the neutralized monomers precipitated, and thus polymerization
may not occur readily, whereas an excessively low degree of neutralization not only
deteriorates the absorbency of the polymer but also endows the polymer with hard-to-handle
properties, such as of elastic rubber.
[0031] The 'first crosslinked polymer' refers to a product obtained by crosslinking polymerization
of the above-described water-soluble ethylene-based unsaturated monomer in the presence
of an internal crosslinking agent, and the 'base polymer powder' refers to a substance
including the first crosslinked polymer. Further, the 'second crosslinked polymer'
refers to a substance obtained by additionally crosslinking the first crosslinked
polymer via a surface crosslinking agent, and accordingly, formed on the base polymer
powder. The surface crosslinking agent will be described below.
[0032] As described above, the superabsorbent polymer of one embodiment may be provided
such that the base polymer powder and the superabsorbent polymer particles have a
relative low aspect ratio by obtaining the base polymer powder by foaming polymerization.
More specifically, the superabsorbent polymer of one embodiment may include a large
number of the superabsorbent polymer particles, for example, 10% by number or more,
10% by number to 80% by number, 10% by number to 70% by number, 10% by number to 60%
by number, or 10% by number to 50% by number of the superabsorbent polymer particle
having an aspect ratio of less than 0.5, the aspect ratio defined as the shortest
diameter/the longest diameter of the superabsorbent polymer particle, based on the
total number of the superabsorbent polymer particles.
[0033] In this regard, the aspect ratio of the base polymer powder and the superabsorbent
polymer particle may be calculated from, for example, the shortest diameter (a) and
the longest diameter (b) which are obtained by analyzing each particle by electron
microscopy, as shown in FIG. 1. From the calculated aspect ratio data of respective
particles, % by number of the particles having the aspect ratio of less than 0.5 may
be calculated. For reference, it is confirmed that the aspect ratios of the base polymer
powder and the superabsorbent polymer particle are equivalent to each other.
[0034] Since the superabsorbent polymer of one embodiment may include the particles having
the low aspect ratio at a predetermined level or more, a number of micropores may
be formed between the base polymer powder and the superabsorbent polymer particles.
When the surface crosslinked layer is formed on the porous particles, a large amount
of water may be rapidly absorbed between the micropores, and therefore, the superabsorbent
polymer of one embodiment may exhibit higher absorption rate and absorption performances
(centrifuge retention capacity, etc.).
[0035] Meanwhile, the above-described superabsorbent polymer of one embodiment may be excellent
in terms of basic absorption performances under no pressure or under pressure, absorption
rate, and liquid permeability, which may be defined by physical properties such as
CRC, AUP, absorbency, SFC, 30-sec absorption rate, surface tension, etc.
[0036] Specifically, centrifuge retention capacity (CRC) of the superabsorbent polymer of
one embodiment for a physiological saline solution (0.9 wt% aqueous solution of sodium
chloride) for 30 minutes may be 25 g/g to 35 g/g, or 26 g/g to 33 g/g. The range of
the centrifuge retention capacity (CRC) may define excellent absorption performance
under no pressure which is exhibited by the superabsorbent polymer of one embodiment.
[0037] The centrifuge retention capacity (CRC) for the physiological saline solution may
be calculated by the following Calculation Formula 1, after immersing the superabsorbent
polymer in the physiological saline solution for 30 minutes:
in Calculation Formula 1, W
0(g) is an initial weight (g) of the superabsorbent polymer,
W1(g) is a weight which is measured after immersing a nonwoven-fabric-made bag including
no superabsorbent polymer in the physiological saline solution at room temperature
for 30 min and draining water off using a centrifuge at 250 G for 3 min, and
W2(g) is a weight which is measured after immersing a nonwoven-fabric-made bag including
the superabsorbent polymer in the physiological saline solution at room temperature
for 30 min and draining water off using a centrifuge at 250 G for 3 min.
[0038] Further, absorbency under pressure (AUP) of 4826 Pa (0.7 psi) of the superabsorbent
polymer of one embodiment for a physiological saline solution (0.9 wt% aqueous solution
of sodium chloride) for 1 hour may be 21 g/g to 27 g/g, or 21.5 g/g to 26 g/g. The
range of the absorbency under pressure (AUP) may define excellent absorption performance
under pressure which is exhibited by the superabsorbent polymer of one embodiment.
[0039] The absorbency under pressure (AUP) may be calculated by the following Calculation
Formula 2, after immersing the superabsorbent polymer in the physiological saline
solution under a pressure of 4826 Pa (0.7 psi) for 1 hour:
in Calculation Formula 2, W
0(g) is an initial weight (g) of the superabsorbent polymer,
W3(g) is the total sum of the weight of the superabsorbent polymer and a weight of an
apparatus capable of providing a load for the superabsorbent polymer, and
W4(g) is the total sum of the weight of the superabsorbent polymer and the weight of
the apparatus capable of providing a load to the superabsorbent polymer, after immersing
the superabsorbent polymer in the physiological saline solution under a load (4826
Pa (0.7 psi)) for 1 hour.
[0040] Further, as the superabsorbent polymer of one embodiment exhibits the centrifuge
retention capacity (CRC) and the absorbency under pressure (AUP) in the above ranges,
the superabsorbent polymer may have absorbency of 46 g/g to 63 g/g or 47 g/g to 60
g/g, which is defined by the following Equation 1:
in Equation 1, CRC is centrifuge retention capacity of the superabsorbent polymer
for the physiological saline solution (0.9 wt% aqueous solution of sodium chloride)
for 30 minutes, and represents centrifuge retention capacity calculated by Calculation
Formula 1, and
[0041] AUP is absorbency under pressure (AUP) of 4826 Pa (0.7 psi) of the superabsorbent
polymer for the physiological saline solution (0.9 wt% aqueous solution of sodium
chloride) for 1 hour, and represents absorbency under pressure calculated by Calculation
Formula 2.
[0042] Accordingly, the superabsorbent polymer of one embodiment may exhibit excellent basic
absorption performances such as absorbency and absorbency under pressure, thereby
being suitably applied to a variety of sanitary materials.
[0043] Further, saline (0.685 wt% aqueous solution of sodium chloride) flow conductivity
(SFC, 10
-7cm
3·s/g) of the superabsorbent polymer of one embodiment may be 30(·10
-7cm
3·s/g) or more, 35(·10
-7cm
3·s/g) or more, 40(·10
-7cm
3·s/g) to 150(·10
-7cm
3·s/g), or 42(·10
-7cm
3·s/g) to 130(·10
-7cm
3·s/g).
[0044] The saline flow conductivity (SFC) may be measured and calculated according to a
method previously known to those skilled in the art, for example, a method disclosed
in column 54 to column 59 of
US Patent No. 5562646.
[0045] Since the superabsorbent polymer may include the base polymer powder which maintains
a high gel strength and may evenly include the surface crosslinked layer having an
excellent strength which is formed by surface crosslinking of the base polymer powder
under particular conditions, the superabsorbent polymer may have an overall high gel
strength, and accordingly, may exhibit more improved saline flow conductivity (SFC)
and excellent liquid permeability.
[0046] Further, the superabsorbent polymer of one embodiment may be prepared/provided by
using a surface crosslinking liquid having a low surface tension described below,
and thus the superabsorbent polymer in itself may have a surface tension of 60 mN/m
to 75 mN/m or 60 mN/m to 73 mN/m.
[0047] The surface tension may be measured, for example, by using a tensiometer at a temperature
of 23±0.5 °C and a humidity of 45±0.5%. A specific method of measuring the surface
tension is described in Examples below.
[0048] The surface tension of the superabsorbent polymer may be a physical property distinguished
from centrifuge retention capacity, absorbency under pressure, liquid permeability,
etc., and may be a measure for evaluating urine leakage from a diaper including the
superabsorbent polymer. The surface tension refers to a surface tension which is measured
with respect to a saline solution after swelling the superabsorbent polymer in the
saline solution. When the surface tension of the superabsorbent polymer is low, there
is a high possibility of urine leakage from diapers including the superabsorbent polymer.
The superabsorbent polymer of one embodiment may have a proper range of surface tension
while maintaining a high liquid permeability, and thus possibility of leakage may
be reduced, thereby producing high-quality sanitary products.
[0049] When the surface tension of the superabsorbent polymer is too low, urine leakage,
that is, rewetting may be increased. When the surface tension is too high, the surface
crosslinked layer may be unevenly formed, and thus physical properties such as liquid
permeability, etc. may deteriorate.
[0050] Meanwhile, the above-described superabsorbent polymer of one embodiment may have
a 30-sec absorption rate of 1.5 mm/min or more, or preferably 1.7 mm/min to 3.0 mm/min,
or more preferably 1.8 mm/min to 2.6 mm/min, when about 0.16 g of the superabsorbent
polymer is swollen under a pressure of 2068 Pa (0.3 psi) by a physiological saline
solution introduced through a mesh in the bottom of a cylindrical cylinder. The 30-sec
absorption rate may be measured and calculated by dividing a change of a height of
an upper plate of a rheometer according to volume expansion of the superabsorbent
polymer by the absorption time (30 sec).
[0051] The superabsorbent polymer may exhibit a high gel strength and excellent liquid permeability
while having a porous structure inside thereof by controlling particle distribution
during the preparation process, and therefore, it may also exhibit excellent absorption
rate defined by the above-described range of 30-sec absorption rate. Accordingly,
the superabsorbent polymer may be preferably used inside sanitary products having
a reduced content of a fibrous material such as pulp, etc.
[0052] Meanwhile, in the above-described superabsorbent polymer of one embodiment, the first
crosslinked polymer included in the base polymer powder may be a polymer obtained
by crosslinking polymerization of the monomer in the presence of a first internal
crosslinking agent of poly(meth)acrylate of polyol selected from the group consisting
of trimethylolpropane tri(meth)acrylate, ethylene glycol di(meth)acrylate, polyethylene
glycol di(meth)acrylate, propylene glycol di(meth)acrylate, polypropylene glycol di(meth)acrylate,
butanediol di(meth)acrylate, butylene glycoldi(meth)acrylate, diethylene glycol di(meth)acrylate,
hexanediol di(meth)acrylate, triethylene glycol di(meth)acrylate, tripropylene glycol
di(meth)acrylate, tetraethylene glycol di(meth)acrylate, dipentaerythritol pentacrylate,
glycerin tri(meth)acrylate and pentaerythritol tetraacrylate; and a second internal
crosslinking agent of allyl(meth)acrylate. By applying two or more kinds of the particular
internal crosslinking agents, the superabsorbent polymer of one embodiment may maintain
a high gel strength even after gel pulverization and pulverization, thereby exhibiting
more excellent liquid permeability and absorption performances under pressure, etc.
[0053] Further, in the above-described superabsorbent polymer, two or more kinds of alkylene
carbonates having 2 to 5 carbon atoms, in which the two or more kinds of the alkylene
carbonates have different carbon numbers, are used as the surface crosslinking agent
during surface crosslinking, which will be described in more detail below. Accordingly,
the superabsorbent polymer includes the surface crosslinked layer including the crosslinked
structure which is obtained via plural kinds of the surface crosslinking agents.
[0054] The surface crosslinking liquid including the surface crosslinking agent and liquid
medium may further include a surfactant, a predetermined polycarboxylic acid-based
copolymer, an aliphatic alcohol having 6 or more carbon atoms, etc. The surface tension
of the surface crosslinking liquid may be achieved in a particular relatively low
range by the use of the plural kinds of the surface crosslinking agents and the additional
components optionally included in the surface crosslinking liquid, thereby finally
preparing and providing the superabsorbent polymer having the above-described physical
properties.
[0055] Meanwhile, the above-described superabsorbent polymer of one embodiment may have
a particle size of 150 µm to 850 µm. More specifically, 95% by weight or more of the
base polymer powder and the superabsorbent polymer including the same may have a particle
size of 150 µm to 850 µm, and fine powder having a particle size of less than 150
µm may be in an amount of less than 5% by weight. In this regard, the particle size
of the superabsorbent polymer may be defined by the longest diameter of the above-described
superabsorbent polymer particle.
[0056] The technical principle by which the superabsorbent polymer satisfying the above-described
physical properties of one embodiment may be prepared is as follows.
[0057] First, a foaming degree is increased by using a foaming agent, etc. during crosslinking
polymerization, thereby forming a water-containing gel polymer including a large number
of micropores and a large surface area. When the water-containing gel polymer is subjected
to gel pulverization and subsequent pulverization, it is highly probable that the
water-containing gel polymer is broken into particles having a low aspect ratio due
to the porous property thereof. Accordingly, a base polymer powder having a high content
of particles having a low aspect ratio may be prepared.
[0058] However, since particles having a low aspect ratio absorb the surface crosslinking
liquid at a relatively high absorption rate, they show the penetration pattern of
the surface crosslinking liquid and the surface crosslinking degree which are different
from those of particles having an aspect ratio close to 1. For this reason, there
is a high possibility that uneven crosslinking may occur, which may cause deterioration
of liquid permeability, etc. However, as a result of continuous studies of the present
inventors, it was found that when the surface tension of the surface crosslinking
liquid is relatively lowered, a penetration depth of the surface crosslinking liquid
into the base polymer powder becomes relatively low, and thus the surface crosslinking
agent may be evenly distributed/penetrated into the entire particles. As a result,
liquid permeability of the superabsorbent polymer may be improved, thereby preparing
the superabsorbent polymer satisfying all the physical properties of one embodiment.
[0059] Based on this technical principle, another embodiment of the present invention provides
a method of preparing the superabsorbent polymer.
[0060] The preparation method may include the steps of:
carrying out a crosslinking polymerization of water-soluble ethylene-based unsaturated
monomers having acidic groups which are at least partially neutralized in the presence
of a foaming agent, a surfactant, and an internal crosslinking agent to form a water-containing
gel polymer including a first cross-linked polymer;
gel-pulverizing, drying, pulverizing, and size-sorting the water-containing gel polymer
to form a base polymer including 10% by number or more of base polymer powder having
an aspect ratio of less than 0.5, the aspect ratio defined as the shortest diameter/the
longest diameter of the base polymer powder; and
carrying out a surface crosslinking of the base polymer by heat treatment in the presence
of a surface crosslinking liquid containing a surface crosslinking agent and a liquid
medium and having a surface tension of 30 mN/m to 50 mN/m at a temperature of 23±0.5
°C and a humidity of 45±0.5%, as measured by a tensiometer, wherein the surface crosslinking
agent includes two or more kinds of alkylene carbonates having 2 to 5 carbon atoms,
in which the two or more kinds of the alkylene carbonates have different carbon numbers,
and wherein the foaming agent is carbonate and added in a concentration of 0.01 parts
by weight to 1.0 parts by weight with respect to 100 parts by weight of the acrylic
acid-based monomer.
[0061] Hereinafter, each step of the preparation method will be described in detail.
[0062] First, the preparation method of another embodiment may include the step of forming
the water-containing gel polymer by crosslinking polymerization. Specifically, this
step is a step of forming the water-containing gel polymer by carrying out thermal
polymerization or photo-polymerization of a monomer composition including the water-soluble
ethylene-based unsaturated monomer and a polymerization initiator in the presence
of the internal crosslinking agent.
[0063] The water-soluble ethylene-based unsaturated monomer included in the monomer composition
is the same as described above.
[0064] Further, the monomer composition may include a polymerization initiator generally
used in the preparation of superabsorbent polymers. For non-limiting example, as the
polymerization initiator, a thermal polymerization initiator or a photo-polymerization
initiator may be used according to the polymerization method. However, even though
the photo-polymerization is performed, a certain amount of heat may be generated by
UV irradiation or the like, and also generated with the polymerization reaction which
is an exothermic reaction. Therefore, the thermal polymerization initiator may be
further included.
[0065] Here, the photo-polymerization initiator may include, for example, one or more compounds
selected from the group consisting of benzoin ether, dialkyl acetophenone, hydroxyl
alkylketone, phenyl glyoxylate, benzyl dimethyl ketal, acyl phosphine, and α-aminoketone.
Among them, as the specific example of acyl phosphine, commercial Lucirin TPO, namely,
2,4,6-trimethyl-benzoyl-trimethyl phosphine oxide may be used. More various photo-polymerization
initiators are well disclosed in "
UV Coatings: Basics, Recent Developments and New Application (Elsevier, 2007)" written
by Reinhold Schwalm, p115, which may be served as a reference.
[0066] Further, the thermal polymerization initiator may include one or more compounds selected
from the group consisting of persulfate-based initiators, azo-based initiators, hydrogen
peroxide, and ascorbic acid. Specific examples of the persulfate-based initiators
may include sodium persulfate (Na
2S
2O
8), potassium persulfate (K
2S
2O
8), ammonium persulfate ((NH
4)
2S
2O
8) or the like. Further, specific examples of the azo-based initiators may include
2,2-azobis(2-amidinopropane) dihydrochloride, 2,2-azobis-(N,N-dimethylene)isobutyramidine
dihydrochloride, 2-(carbamoylazo)isobutylonitril, 2,2-azobis(2-[2-imidazolin-2-yl]propane)dihydrochloride,
4,4-azobis-(4-cyanovaleric acid) or the like. More various thermal polymerization
initiators are well-disclosed in "
Principle of Polymerization" written by Odian, (Wiley, 1981), p 203, which may be served as a reference.
[0067] The polymerization initiator may be added at a concentration of about 0.001% by weight
to 1% by weight, based on the monomer composition. That is, if the concentration of
the polymerization initiator is too low, the polymerization rate becomes low, and
thus a large amount of residual monomers may be undesirably extracted from the final
product. On the contrary, if the concentration of the polymerization initiator is
too high, the polymer chains constituting the network becomes short, and thus the
content of water-soluble components is increased and physical properties of the polymer
may deteriorate such as a reduction in absorbency under pressure.
[0068] Meanwhile, the monomer composition may include a crosslinking agent ("internal crosslinking
agent") for improving physical properties of the polymer by polymerization of the
water-soluble ethylene-based unsaturated monomer. The crosslinking agent is used for
internal crosslinking of the water-containing gel polymer, and the crosslinking agent
is separately used, independent of a "surface crosslinking agent" described below.
[0069] Particularly, in the preparation method of another embodiment, two or more kinds
of the above-described internal crosslinking agents, e.g., the first internal crosslinking
agent of poly(meth)acrylate of polyol and the second internal crosslinking agent of
allyl(meth)acrylate may be used together to obtain the water-containing gel polymer.
[0070] More specifically, the first internal crosslinking agent may include one or more
selected from the group consisting of trimethylolpropane tri(meth)acrylate, ethylene
glycol di(meth)acrylate, polyethylene glycol di(meth)acrylate, propylene glycol di(meth)acrylate,
polypropylene glycol di(meth)acrylate, butanediol di(meth)acrylate, butylene glycoldi(meth)acrylate,
diethylene glycol di(meth)acrylate, hexanediol di(meth)acrylate, triethylene glycol
di(meth)acrylate, tripropylene glycol di(meth)acrylate, tetraethylene glycol di(meth)acrylate,
dipentaerythritol pentacrylate, glycerin tri(meth)acrylate, and pentaerythritol tetraacrylate,
and the second internal crosslinking agent may include allyl(meth)acrylate, allylacrylate,
etc.
[0071] Further, a total content of the first and second internal crosslinking agents may
be 0.01 parts by weight to 2 parts by weight, or 0.05 to 1.8 parts by weight, based
on 100 parts by weight of the monomer composition including the internal crosslinking
agents and monomer. Further, the first internal crosslinking agent : the second internal
crosslinking agent may be used in a weight ratio of 1 : 1 to 10 : 1. The superabsorbent
polymer satisfying the physical properties of one embodiment may be effectively obtained
by controlling the type and content range of the internal crosslinking agents while
controlling a water content of the water-containing gel polymer, described below.
However, when the content of the internal crosslinking agents is too large, basic
absorption performances of the superabsorbent polymer may deteriorate.
[0072] Meanwhile, the above-described monomer composition further includes a foaming agent.
When the polymerization process is performed by a foaming polymerization process in
the presence of the foaming agent, a large number of particles having a low aspect
ratio may be formed, and the base polymer powder and the superabsorbent polymer particle
having the above-described particle distribution may be obtained.
[0073] The foaming agent serves to foam during polymerization to form pores inside the water-containing
gel polymer, thereby forming a large number of particles having a low aspect ratio
and increasing the surface area. The foaming agent is carbonate, for example, sodium
bicarbonate, sodium carbonate, potassium bicarbonate, potassium carbonate, calcium
bicarbonate, calcium carbonate, magnesium bicarbonate, or magnesium carbonate.
[0074] Further, the foaming agent is added at a concentration of about 0.01 parts by weight
to about 1.0 parts by weight, or preferably about 0.03 parts by weight to about 0.7
parts by weight, or more preferably about 0.05 parts by weight to about 0.6 parts
by weight with respect to 100 parts by weight of the acrylic acid-based monomer. When
the use of the foaming agent exceeds 1.0 parts by weight, the production process is
difficult due to excessive foaming, and the density of the superabsorbent polymer
becomes small due to excessive pore formation, which may cause problems in distribution
and storage. Further, when the use of the foaming agent is less than 0.01 parts by
weight, the role of the foaming agent may be insignificant.
[0075] The monomer composition may further include a foam stabilizer to optimize pore formation
by the foaming agent. The foam stabilizer plays a role in maintaining the shape of
the pores produced by the foaming agent while uniformly distributing pores throughout
the polymer, thereby more effectively forming particles having a low aspect ratio
and increasing the surface area of the polymer.
[0076] As the foam stabilizer, any component which has been previously used as the foam
stabilizer in the foaming polymerization of the superabsorbent polymer may be used.
For example, cationic, anionic, or non-ionic surfactants may be used.
[0077] The foam stabilizer may be added at a concentration of 0.001 parts by weight to 0.1
parts by weight with respect to 100 parts by weight of the acrylic acid-based monomer.
When the concentration of the foam stabilizer is too low, the role of stabilizing
the foam is insignificant and it is difficult to achieve the effect of improving the
absorption rate. On the contrary, when the concentration is too high, the surface
tension of the superabsorbent polymer is lowered, and water leakage may occur in the
diaper.
[0078] Additionally, the monomer composition may further include a thickener, a plasticizer,
a preservation stabilizer, an antioxidant, etc., as needed.
[0079] The monomer composition may be prepared in the form of a solution in which the raw
materials such as the above-described monomer, polymerization initiator, internal
crosslinking agent, etc. are dissolved in a solvent.
[0080] In this regard, as the applicable solvent, any solvent may be used without limitations
in the constitution as long as it is able to dissolve the above raw materials. For
example, as the solvent, water, ethanol, ethylene glycol, diethylene glycol, triethylene
glycol, 1,4-butanediol, propylene glycol, ethylene glycol monobutyl ether, propylene
glycol monomethyl ether, propylene glycol monomethyl ether acetate, methyl ethyl ketone,
acetone, methyl amyl ketone, cyclohexanone, cyclopentanone, diethylene glycol monomethyl
ether, diethylene glycol ethylether, toluene, xylene, butyrolactone, carbitol, methyl
cellosolve acetate, N,N-dimethylacetamide, or a mixture thereof may be used.
[0081] Further, formation of the water-containing gel polymer by polymerization of the monomer
composition may be performed by a common polymerization method, and the process is
not particularly limited. For non-limiting example, the polymerization method is largely
classified into thermal polymerization and photo-polymerization according to a polymerization
energy source. The thermal polymerization may be carried out in a reactor like a kneader
equipped with agitating spindles, and the photo-polymerization may be carried out
in a reactor equipped with a movable conveyor belt.
[0082] For example, the monomer composition is injected to a reactor like a kneader equipped
with the agitating spindles, and thermal polymerization is performed by providing
hot air thereto or by heating the reactor so as to obtain the water-containing gel
polymer. In this regard, the water-containing gel polymer may have a size of centimeters
or millimeters when it is discharged from an outlet of the reactor, according to the
type of agitating spindles equipped in the reactor. Specifically, the water-containing
gel polymer may be obtained in various forms according to a concentration of the monomer
composition fed thereto, a feeding speed or the like, and the water-containing gel
polymer having a (weight average) particle size of 2 mm to 50 mm may be generally
obtained.
[0083] For another example, when the monomer composition is subjected to photo-polymerization
in a reactor equipped with a movable conveyor belt, the water-containing gel polymer
may be obtained in a sheet-type. In this regard, the thickness of the sheet may vary
according to the concentration of the monomer composition fed thereto and the feeding
speed. The sheet is preferably controlled to have a thickness of 0.5 cm to 10 cm in
order to assure the production speed while allowing the entire sheet to be uniformly
polymerized.
[0084] Meanwhile, after forming the water-containing gel polymer by the above-described
crosslinking polymerization, the water-containing gel polymer with a controlled water
content is subjected to gel pulverization.
[0085] A pulverizer applicable in the gel pulverization step may have no limitation in the
constitution, but may specifically include any one selected from the group consisting
of a vertical pulverizer, a turbo cutter, a turbo grinder, a rotary cutter mill, a
cutter mill, a disc mill, a shred crusher, a crusher, a chopper, and a disc cutter,
but is not limited thereto.
[0086] The gel pulverization of the water-containing gel polymer may be performed such that
the diameter of the water-containing gel polymer is 0.01 mm to 50 mm, or 0.01 mm to
30 mm. In other words, the water-containing gel polymer is preferably pulverized into
particles of 50 mm or less in order to increase the drying efficiency. However, when
excessive pulverization is performed, an aggregation phenomenon between particles
may occur, and thus the water-containing gel polymer is preferably pulverized into
particles of 0.01 mm or more.
[0087] Further, when the gel pulverization of the water-containing gel polymer is performed,
the water-containing gel polymer may stick to the surface of the gel pulverizer because
it has a relatively low water content. In order to minimize this phenomenon, steam,
water, a surfactant, an anti-agglomeration agent (e.g., clay, silica, etc.), a persulfate-based
initiator, an azo-based initiator, hydrogen peroxide, a thermal polymerization initiator,
an epoxy-based crosslinking agent, a diol-based crosslinking agent, a crosslinking
agent including 2-functional or 3 or more-functional acrylate, or a mono-functional
crosslinking agent including a hydroxyl group may be added to the water-containing
gel polymer, if necessary.
[0088] After the above-described gel pulverization, the water-containing gel polymer may
be dried. The drying may be performed at a temperature of 120°C to 250°C, preferably
140°C to 200°C, and more preferably 150°C to 200°C. In this regard, the drying temperature
is defined as a temperature of a heating medium provided for drying, or an internal
temperature of a drying reactor including the heating medium and the polymer during
the drying process. If the drying temperature is low and therefore the drying time
becomes long, the process efficiency may be decreased. In order to prevent this problem,
the drying temperature is preferably 120°C or higher. In addition, when the drying
temperature is higher than necessary, the surface of the water-containing gel polymer
is excessively dried, and thus there is a concern about generation of fine particles
during the subsequent pulverization process and deterioration of the physical properties
of the finally formed polymer. In order to prevent this problem, therefore, the drying
temperature is preferably 250°C or lower.
[0089] In this regard, the drying time in the drying step is not particularly limited, but
may be controlled to 20 minutes to 90 minutes at the above drying temperature, in
consideration of process efficiency and physical properties of the polymer.
[0090] The drying may be carried out by using a general medium, and for example, the pulverized
water-containing gel polymer may be supplied with hot air, or irradiated with infrared
rays, microwaves, ultraviolet rays, or the like.
[0091] The drying as above is performed such that the dried polymer may preferably have
the water content of about 0.1% by weight to about 10% by weight. In other words,
if the water content of the dried polymer is less than 0.1% by weight, production
costs may be increased and degradation of the crosslinked polymer may undesirably
occur due to excessive drying. If the water content of the dried polymer is more than
10% by weight, the dried polymer adheres in the subsequent process, which may undesirably
interfere with a transport path.
[0092] After drying, the dried polymer may be pulverized, thereby controlling the particle
size and surface area of the polymer in a proper range. The pulverization may be performed
such that the particle size of the pulverized polymer is 150 µm to 850 µm. In this
regard, the particle size is also defined by the longest diameter of each polymer
particle, and the same applies hereinafter.
[0093] A milling device applicable herein may include a pin mill, a hammer mill, a screw
mill, a roll mill, a disc mill, a jog mill, or the like which is commonly used.
[0094] In order to manage physical properties of the superabsorbent polymer finally prepared,
a step of selectively size-sorting particles having a particle size of 150 µm to 850
µm from the polymer particles obtained through the pulverization step may be further
performed.
[0095] Meanwhile, after preparing the base polymer powder through the above-described size-sorting
process, the base polymer powder is surface-crosslinked by heat treatment in the presence
of a surface crosslinking agent, thereby forming the superabsorbent polymer particles.
The surface crosslinking is to induce a crosslinking reaction on the surface of the
base polymer powder in the presence of the surface crosslinking agent. Through this
surface crosslinking, a surface-modified layer (surface-crosslinked layer) may be
formed on the surface of the base polymer powder.
[0096] More specifically, in the above-described preparation method of another embodiment,
surface crosslinking may be performed by heat treatment using a surface crosslinking
liquid including the surface crosslinking agent and liquid medium and having a surface
tension of 25 mN/m to 50 mN/m or 30 mN/m to 47 mN/m at a temperature of 23±0.5 °C
and a humidity of 45±0.5%.
[0097] As such, when the surface crosslinking liquid having the relatively low surface tension
is used, surface crosslinking evenly occurs although the particle shape is relatively
uneven (a number of particles having a low aspect ratio are included), and therefore,
the surface crosslinked layer having excellent crosslinking degree and strength may
be uniformly formed, and as relative to absorbency of the superabsorbent polymer,
its absorbency under pressure and liquid permeability may be more improved. However,
when the surface tension is too low, rewetting of sanitary products may be increased.
When a surface crosslinking liquid having a high surface tension is used, the surface
crosslinked layer is unevenly formed, and thus physical properties such as absorbency
under pressure and liquid permeability may deteriorate, as relative to absorbency.
[0098] As described above, in order to obtain the surface crosslinking liquid having such
a particular surface tension, two or more kinds of alkylene carbonates having 2 to
5 carbon atoms, in which the two or more kinds of the alkylene carbonates have different
carbon numbers, are used as the surface crosslinking agent.
[0099] Further, in order to more effectively achieve the surface tension, the surface crosslinking
liquid including the surface crosslinking agent and the liquid medium may further
include a surfactant, a polycarboxylic acid-based copolymer having repeating units
represented by the following Chemical Formulae 1-a and 1-b, or aliphatic alcohol having
6 or more carbon atoms, optionally. As such, due to use of the plural kinds of the
surface crosslinking agents, and optionally, additional components included in the
surface crosslinking liquid, the surface crosslinking liquid may have the surface
tension in the particular relatively low range, thereby preparing the superabsorbent
polymer having the above-described physical properties of one embodiment:
in Chemical Formulae 1-a and 1-b, R
1, R
2 and R
3 are each independently hydrogen or an alkyl group having 1 to 6 carbon atoms, RO
is an oxyalkylene group having 2 to 4 carbon atoms, M
1 is hydrogen or a monovalent metal or non-metal ion, X is -COO-, an alkyloxy group
having 1 to 5 carbon atoms, or an alkyldioxy group having 1 to 5 carbon atoms, m is
an integer of 1 to 100, n is an integer of 1 to 1000, and p is an integer of 1 to
150, wherein when p is two or more, two or more of the repeating -RO- may be the same
as or different from each other.
[0100] In the surface crosslinking process, plural kinds of alkylene carbonate having 2
to 5 carbon atoms are used as the surface crosslinking agent, and more suitable examples
thereof may include ethylene carbonate, propylene carbonate, butylene carbonate, trimethylene
carbonate, glycerol carbonate, etc.
[0101] In this regard, a content of the surface crosslinking agent may be appropriately
controlled according to the kind or reaction conditions of the crosslinking agent,
and preferably, the content may be controlled from 0.001 part by weight to 5 parts
by weight with respect to 100 parts by weight of the base polymer powder. When the
content of the surface crosslinking agent is too low, surface modification does not
occur properly, and thus physical properties of the final polymer may deteriorate.
On the contrary, when the surface crosslinking agent is excessively used, excessive
surface crosslinking reaction may occur, leading to deterioration in the basic absorbency
of the polymer, undesirably.
[0102] Further, in order to control the surface tension range of the surface crosslinking
liquid, the surface crosslinking liquid may further include a surfactant, and the
kind of the surfactant is not particularly limited. In consideration of the kind of
the liquid medium included in the surface crosslinking liquid, a nonionic surfactant,
an anionic surfactant, or a cationic surfactant may be appropriately selected and
used. Accordingly, the surface tension of the surface crosslinking liquid may be further
controlled in the above-described range.
[0103] For another example, the surface crosslinking liquid may further include the polycarboxylic
acid-based copolymer having repeating units represented by the following Chemical
Formulae 1-a and 1-b. The polycarboxylic acid-based copolymer is disclosed in Patent
No.
1684649, and a preparation method thereof is apparent to those skilled in the art.
[0104] The polycarboxylic acid-based copolymer may be included in an amount of 0.001 part
by weight to 5 parts by weight in the surface crosslinking liquid with respect to
100 parts by weight of the base polymer powder, thereby further controlling the surface
tension of the surface crosslinking liquid in the above-described range.
[0105] Additionally, as another means for controlling the surface tension of the surface
crosslinking liquid, an aliphatic alcohol having 6 or more carbon atoms may be further
included in the liquid medium of the surface crosslinking liquid, together with a
polar solvent such as water or alcohol.
[0106] According to an embodiment, aliphatic alcohol having 6 or more carbon atoms may be
exemplified by C6 to C20 primary, secondary, or tertiary alcohols, and preferably,
C6 to C16 primary alcohols. More preferably, one or more selected from the group consisting
of stearyl alcohol, lauryl alcohol, and cetyl alcohol may be used, but is not limited
thereto.
[0107] A content of the aliphatic alcohol having 6 or more carbon atoms may be about 0.001
part by weight to about 2 parts by weight, or about 0.01 part by weight to about 1
part by weight, preferably about 0.01 part by weight to about 1 part by weight, more
preferably about 0.05 parts by weight to about 0.8 parts by weight with respect to
100 parts by weight of the pulverized polymer, i.e., the base polymer powder.
[0108] Meanwhile, the surface crosslinking liquid may further include water and/or a hydrophilic
organic solvent (e.g., an alcohol-based polar organic solvent such as methanol, etc.)
as the liquid medium, together with the above-described components. In this regard,
water and the hydrophilic organic solvent may be applied by controlling its addition
ratio with respect to 100 parts by weight of the base polymer powder, for the purpose
of inducing uniform distribution of the surface crosslinking liquid, preventing the
aggregation phenomenon of the base polymer powder, and optimizing the surface penetration
depth of the surface crosslinking agent.
[0109] With regard to the method of adding the above-described surface crosslinking liquid
to the base polymer powder, there is no particular limitation in the constitution.
For example, a method of adding and mixing the surface crosslinking liquid and the
base polymer powder in a reactor, a method of spraying the surface crosslinking liquid
onto the base polymer powder, or a method of continuously feeding the base polymer
powder and the surface crosslinking liquid to a mixer which is continuously operated
may be used.
[0110] The base polymer powder to which the surface crosslinking liquid is applied is subjected
to surface crosslinking reaction at a maximum reaction temperature of 140°C to 200°C
or 170°C to 195°C for 5 minutes to 60 minutes, 10 minutes to 50 minutes, or 20 minutes
to 45 minutes. More specifically, the surface-crosslinking step may be performed by
heat treatment by raising an initial temperature of 20°C to 130°C or 40°C to 120°C
to the maximum reaction temperature for 10 minutes or longer, or 10 minutes to 30
minutes, and maintaining the maximum temperature for 5 minutes to 60 minutes.
[0111] By satisfying these surface crosslinking process conditions (in particular, heating
conditions and reaction conditions at the maximum reaction temperature), the superabsorbent
polymer satisfying the physical properties of one embodiment may be more effectively
prepared.
[0112] A means for raising the temperature for surface crosslinking reaction is not particularly
limited. Heating may be performed by providing a heating medium or by directly providing
a heat source. In this regard, the kind of the heating medium applicable may be a
hot fluid such as steam, hot air, hot oil or the like, but is not limited thereto.
The temperature of the heating medium provided may be properly controlled, considering
the means of the heating medium, the heating rate, and the target temperature. Meanwhile,
as the heat source provided directly, an electric heater or a gas heater may be used,
but the present invention is not limited to these examples.
[0113] The superabsorbent polymer obtained according to the above-described preparation
method may satisfy more improved liquid permeability and absorption rate while maintaining
excellent absorption performances such as centrifuge retention capacity and absorbency
under pressure, thereby satisfying the physical properties of one embodiment. Accordingly,
the superabsorbent polymer may be appropriately applied to sanitary products such
as diapers, particularly, ultra-thin sanitary products with reduced pulp content.
[Effect of the Invention]
[0114] The superabsorbent polymer according to the present invention may exhibit more improved
absorption rate and liquid permeability while maintaining excellent basic absorption
performances, thereby being preferably applied to sanitary products such as thinner
diapers, etc.
[Brief Description of Drawings]
[0115] FIG. 1 is an electron microscopic image showing definition of an aspect ratio of
a superabsorbent polymer particle in a superabsorbent polymer of one embodiment and
an exemplary method of measuring the same.
[Detailed Description of the Embodiments]
[0116] Hereinafter, preferred Examples will be provided for better understanding of the
present invention. However, these Examples are for illustrative purposes only, and
the present invention is not intended to be limited by these Examples.
Example 1
[0117] As a manufacturing apparatus of a superabsorbent polymer, a continuous manufacturing
apparatus consisting of a polymerization process, a water-containing gel pulverizing
process, a drying process, a pulverizing process, a size-sorting process, a surface
crosslinking process, a cooling process, a size-sorting process, and a transportation
process connecting respective steps was used.
(Step 1)
[0118] 100 parts by weight of acrylic acid was mixed with 0.4 parts by weight of polyethylene
glycol diacrylate (a weight average molecular weight of ~ 500 g/mol) and allyl(meth)acrylate
as internal crosslinking agents, 0.1 part by weight of sodium bicarbonate as a foaming
agent, 0.01 parts by weight of sodium lauryl sulfate as a surfactant, and 0.01 part
by weight of phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide as a photoinitiator
to prepare a monomer solution. Subsequently, while continuously feeding the monomer
solution by a metering pump, 160 parts by weight of a 24 wt% aqueous solution of sodium
hydroxide was continuously subjected to line mixing to prepare an aqueous monomer
solution. Further, 6 parts by weight of a 4 wt% aqueous solution of sodium persulfate
was continuously subjected to line mixing, and then continuously fed into a continuous
polymerization reactor having a planar polymerization belt with a dam at each end.
Thereafter, UV was irradiated to prepare a water-containing gel.
(Step 2)
[0119] The water-containing gel was cut to have an average size of about 300 mm or less,
and then introduced into a pulverizer (equipped with a perforated plate including
a plurality of pores having a diameter of 10 mm), followed by pulverization under
respective conditions.
(Step 3)
[0120] Subsequently, the water-containing gel pulverized in the step 2 was dried in a drier
capable of moving the air volume up and down. The water-containing gel was uniformly
dried by allowing hot air of 180°C to flow upward from downward for 15 minutes so
that the water content of the dried powder was about 2% or less, and again allowing
the hot air to flow downward from upward for 15 minutes.
(Step 4)
[0121] The polymer dried in step 3 was pulverized by a pulverizer and then size-sorted to
obtain a base polymer having a size of 150 µm to 850 µm.
(Step 5)
[0122] Then, 1 g of ethylene carbonate and 1 g of propylene carbonate were mixed in 4 g
of water to prepare a surface crosslinking liquid. The surface tension of the surface
crosslinking liquid was measured as 45 mN/m.
[0123] 6 g of the surface crosslinking liquid was sprayed onto 100 g of the base polymer
powder prepared in step 4, and stirred at room temperature to allow uniform distribution
of the surface crosslinking liquid on the base polymer powder. Subsequently, the base
polymer powder mixed with the surface crosslinking liquid was introduced into a surface
crosslinking reactor to perform surface crosslinking reaction.
[0124] In the surface crosslinking reactor, the base polymer powder was confirmed to be
gradually heated at an initial temperature of about 80°C, and 30 minutes later, allowed
to reach the maximum reaction temperature of 190°C. After reaching the maximum reaction
temperature, the reaction was further allowed for 15 minutes. Then, a sample of the
finally prepared superabsorbent polymer was taken. After the surface crosslinking
process, the superabsorbent polymer was size-sorted using an ASTM standard sieve to
prepare a superabsorbent polymer having a particle size of 150 µm to 850 µm of Example
1.
[0125] The base polymer and the superabsorbent polymer obtained by the above method were
analyzed by an electron microscopic image (see FIG. 1, etc.), and an aspect ratio
(a/b) of each base polymer powder and superabsorbent polymer particle was calculated.
A ratio (% by number) of particles having an aspect ratio of less than 0.5 in the
entire base polymer powder and superabsorbent polymer particles was measured. As a
result, it was confirmed that the ratio of the particles having an aspect ratio of
less than 0.5 in the corresponding base polymer powder and superabsorbent polymer
particle was about 10% by number.
Example 2
[0126] A superabsorbent polymer of Example 2 was prepared in the same manner as in Example
1, except that 0.15 parts by weight of sodium bicarbonate was used as the foaming
agent. The base polymer/superabsorbent polymer obtained by the above method were analyzed
by an electron microscopic image, and a ratio (% by number) of particles having an
aspect ratio of less than 0.5 in the entire base polymer powder and superabsorbent
polymer particles was measured. As a result, it was confirmed that the ratio of the
particles having an aspect ratio of less than 0.5 in the corresponding base polymer
powder and superabsorbent polymer particle was about 33% by number.
Example 3
[0127] A superabsorbent polymer of Example 3 was prepared in the same manner as in Example
1, except that 0.2 parts by weight of sodium bicarbonate was used as the foaming agent.
The base polymer/superabsorbent polymer obtained by the above method were analyzed
by an electron microscopic image, and a ratio (% by number) of particles having an
aspect ratio of less than 0.5 in the entire base polymer powder and superabsorbent
polymer particles was measured. As a result, it was confirmed that the ratio of the
particles having an aspect ratio of less than 0.5 in the corresponding base polymer
powder and superabsorbent polymer particle was about 45% by number.
[0128] The subsequent surface crosslinking process was performed in the same manner as in
Example 1 to prepare the superabsorbent polymer having a particle size of 150 µm to
850 µm of Example 3.
Example 4
[0129] A superabsorbent polymer of Example 4 was prepared in the same manner as in Example
3, except that 0.02 g of polyoxyethylenesorbitan monopalmitate as a lubricant was
added to the surface crosslinking liquid in step 5.
Example 5
[0130] A superabsorbent polymer of Example 5 was prepared in the same manner as in Example
3, except that 0.3 g of aliphatic alcohol (monostearyl alcohol) as a lubricant was
added to the surface crosslinking liquid in step 5.
Example 6
[0131] A superabsorbent polymer of Example 6 was prepared in the same manner as in Example
1, except that 0.1 g of polycarboxylic acid-based copolymer as a lubricant which was
prepared in the same manner as in Preparation Example 1 of Patent No.
1684649 was added to the surface crosslinking liquid in step 5.
Example 7
[0132] A superabsorbent polymer of Example 7 was prepared in the same manner as in Example
3, except that a surface crosslinking liquid prepared by mixing 1 g of trimethylene
carbonate and 1 g of propylene carbonate in 4 g of water was used as the surface crosslinking
liquid in step 5.
Comparative Example 1
[0133] A base polymer of Comparative Example 1 was prepared in the same manner as in Example
1, except that sodium bicarbonate was not used as the foaming agent in step 1. The
base polymer obtained by this method was analyzed by an electron microscopic image,
and a ratio (% by number) of particles having an aspect ratio of less than 0.5 in
the entire base polymer powder was measured. As a result, it was confirmed that the
ratio of the particles having an aspect ratio of less than 0.5 in the corresponding
base polymer powder was about 5% by number.
Comparative Example 2
[0134] A superabsorbent polymer of Comparative Example 2 was prepared in the same manner
as in Comparative Example 1, except that 100 parts by weight of the prepared base
polymer powder and 5 g of a surface crosslinking liquid prepared by mixing 1 g of
ethylene carbonate in 4 g of water were used. The surface tension of the surface crosslinking
liquid was measured as 51 mN/m.
Comparative Example 3
[0135] A superabsorbent polymer of Comparative Example 3 was prepared in the same manner
as in Example 1, except that 100 parts by weight of the prepared base polymer powder
and 5 g of a surface crosslinking liquid prepared by mixing 1 g of ethylene carbonate
in 4 g of water were used.
Comparative Example 4
[0136] A superabsorbent polymer of Comparative Example 4 was prepared in the same manner
as in Example 3, except that 100 parts by weight of the prepared base polymer powder
and 5 g of a surface crosslinking liquid prepared by mixing 1 g of ethylene carbonate
in 4 g of water were used.
Comparative Example 5
[0137] Preparation and drying of the water-containing gel polymer were performed according
to a method described in Preparation Example of
Korean Patent Publication No. 2015-0132035. Thereafter, a base polymer was prepared and subjected to surface crosslinking according
to a method described in Example 1 of
Korean Patent Publication No. 2015-0132035, thereby preparing a superabsorbent polymer of Comparative Example 5.
Experimental Example
[0138] Physical properties of the respective superabsorbent polymers prepared in Examples
and Comparative Examples, and all physical properties during the preparation processes
were measured and evaluated by the following methods.
(1) Measurement of aspect ratio and particle distribution of base polymer powder and
superabsorbent polymer particle
[0139] The shortest diameter (a) and longest diameter (b) of each powder/particle were calculated
by electron microscopy as in FIG. 1, and an aspect ratio of each powder/particle was
calculated therefrom. A ratio (% by number) of powder/particles having an aspect ratio
of less than 0.5 in the entire powder/particles obtained in each of Examples/Comparative
Examples was calculated.
(2) Centrifuge retention capacity (CRC)
[0140] The centrifuge retention capacity (CRC) by water absorption capacity under no load
was measured in accordance with EDANA (European Disposables and Nonwovens Association)
standard test method WSP 241.3. After uniformly introducing W
0(g) (about 0.2 g) of the superabsorbent polymer in a nonwoven fabric-made bag and
sealing the same, it was immersed in a physiological saline solution composed of 0.9
wt% aqueous solution of sodium chloride at room temperature. After 30 minutes, the
bag was dehydrated by using a centrifuge at 250 G for 3 minutes, and then the weight
W
2(g) of the bag was measured. Further, after carrying out the same operation without
using the superabsorbent polymer, the weight W
1(g) of the bag was measured. CRC (g/g) was calculated by using the obtained weight
values according to the following Calculation Formula 1, thereby confirming the centrifuge
retention capacity.
(3) Absorbency under pressure (AUP)
[0141] The absorbency under pressure (AUP) of the superabsorbent polymers of Examples and
Comparative Examples was measured in accordance with EDANA (European Disposables and
Nonwovens Association) standard test method WSP 242.3.
[0142] First, a 400 mesh stainless steel net was installed in the cylindrical bottom of
a plastic having an internal diameter of 60 mm. W
0(g, 0.90 g) of each of the superabsorbent polymers of Examples 1 to 6 and Comparative
Examples 1 to 4 was uniformly scattered on the steel net under conditions of temperature
of 23±2°C and relative humidity of 45%, and a piston which can uniformly provide a
load of 4.83 kPa (0.7 psi) was put thereon. The external diameter of the piston was
slightly smaller than 60 mm, there was no gap between the cylindrical internal wall
and the piston, and the jig-jog of the cylinder was not interrupted. At this time,
the weight W
3(g) of the apparatus was measured.
[0143] After putting a glass filter having a diameter of 125 mm and a thickness of 5 mm
in a Petri dish having a diameter of 150 mm, a physiological saline solution composed
of 0.90 wt% sodium chloride was poured in the dish until the surface level became
equal to the upper surface of the glass filter. The measuring apparatus was put on
the glass filter and the solution was absorbed under a load for about 1 hour. After
1 hour, the weight W
4(g) was measured after lifting up the measuring apparatus.
[0144] Using the respective weights thus obtained, AUP(g/g) was calculated according to
the following Calculation Formula 2, thereby confirming the absorbency under pressure.
in Calculation Formula 2, W
0(g) is an initial weight (g) of the superabsorbent polymer,
W3(g) is the total sum of a weight of the superabsorbent polymer and a weight of the
apparatus capable of providing a load for the superabsorbent polymer, and
W4(g) is the total sum of a weight of the superabsorbent polymer and a weight of the
apparatus capable of providing a load to the superabsorbent polymer, after immersing
the superabsorbent polymer in a physiological saline solution under a load (4826 Pa
(0.7 psi)) for 1 hour.
(4) Saline flow conductivity (SFC)
[0145] The saline flow conductivity (SFC) was measured and calculated according to the methods
disclosed in columns 54 to 59 of
US Patent No. 5562646.
(5) 30-sec absorption rate
[0146] 30-sec absorption rate and porosity were measured by swelling about 0.16 g of the
superabsorbent polymer in a physiological saline solution fed through a mesh in the
bottom of a cylindrical cylinder under a pressure of 2068 Pa (0.3 psi). A change of
a height of an upper plate of a rheometer according to volume expansion of the superabsorbent
polymer was measured in real time, and from a value obtained by dividing the height
of the upper plate at 30 sec by the absorption time (30 sec), the 30-sec absorption
rate was measured and calculated. Further, porosity was calculated by the following
method: when swelling of the superabsorbent polymer was completed, the total volume
inside the cylinder (final absorption height * the bottom area of the cylindrical
cylinder) was calculated, and from this value, the amount of the physiological saline
solution absorbed by the superabsorbent polymer which was measured by a water content
meter was subtracted.
(6) Surface tension of surface crosslinking liquid and superabsorbent polymer
[0147] All procedures were carried out in a constant temperature and humidity room (temperature
of 23±0.5°C, relative humidity of 45±0.5%).
[0148] First, the surface crosslinking liquid was pipetted and transferred to another clean
cup, and then the surface tension of the surface crosslinking liquid was measured
by using a tensiometer (surface tensionmeter Kruss K11/K100).
[0149] Next, the surface tension of the superabsorbent polymer was measured as follows.
150 g of physiological saline composed of 0.9 wt% sodium chloride was put in a 250
mL beaker, and directly stirred with a magnetic bar. 1.0 g of the superabsorbent polymer
was added to the solution under stirring, and stirred for 3 minutes. Stirring was
stopped and the swollen superabsorbent polymer was allowed to settle to the bottom
for 15 minutes or longer.
[0150] Then, the supernatant (the solution just below the surface) was pipetted and transferred
to another clean cup and measured using a tensiometer (surface tensionmeter Kruss
K11/K100).
[0151] The values of physical properties of Examples 1 to 7 and Comparative Examples 1 to
5 which were measured by the above methods are summarized and shown in Table 1 below.
[Table 1]
|
Surface tension (surface crosslinki ng liquid) |
Particle distributi on (aspect ratio of less than 0.5) |
CR C |
AU P |
Absorbe ncy |
SFC |
30-sec absorpti on rate |
Surface tension (superabsor bent polymer) |
Unit |
mN/m |
% by number |
g/g |
g/g |
g/g |
·10-7cm3·s /g |
mm/min |
mN/m |
Example 1 |
45 |
10 |
28. 3 |
25. 0 |
53.3 |
53 |
1.9 |
70 |
Example 2 |
45 |
33 |
28. 0 |
24. 6 |
52.6 |
50 |
2.2 |
68 |
Example 3 |
45 |
45 |
26. 4 |
24. 8 |
51.2 |
49 |
2.4 |
66 |
Example 4 |
33 |
45 |
27. 9 |
25. 4 |
53.3 |
42 |
2.4 |
60 |
Example 5 |
33 |
45 |
27. 0 |
25. 3 |
52.3 |
50 |
2.4 |
60 |
Example 6 |
42 |
10 |
27. 6 |
24. 5 |
52.1 |
50 |
1.9 |
69 |
Example 7 |
45 |
45 |
27. 7 |
24. 1 |
51.8 |
43 |
1.9 |
66 |
Comparat ive Example 1 |
45 |
5 |
27. 7 |
25. 7 |
53.4 |
46 |
1.4 |
70 |
Comparat ive Example 2 |
51 |
5 |
28. 7 |
24. 7 |
53.4 |
44 |
1.4 |
71 |
Comparat ive Example 3 |
51 |
10 |
27. 8 |
23. 2 |
51 |
20 |
1.7 |
69 |
Comparat ive Example 4 |
51 |
45 |
27. 7 |
23. 0 |
50.7 |
25 |
2.4 |
68 |
Comparat ive Example 5 |
45 |
2 |
33. 1 |
24. 2 |
57.3 |
5 |
0 |
70 |
[0152] Referring to Table 1, Examples 1 to 7 were confirmed to satisfy predetermined particle
distributions and to exhibit excellent liquid permeability defined as 35 (·10
-7cm
3·s/g) or more. It was also confirmed that Examples 1 to 7 showed excellent basic absorption
performances defined by absorbency, etc., and also showed optimized particle distributions
while having excellent liquid permeability, thereby showing excellent absorption rate
defined by 30-sec absorption rate.
[0153] In contrast, one or more of liquid permeability and absorption rate were poor in
Comparative Examples 1 to 5, as compared with Examples.